1. Field of the Invention
The present invention relates to switched capacitor analog to digital converters (ADCs) and particularly relates to a switched capacitor filter for use in establishing reference voltages for such ADCs.
2. Background of the Invention
Switched capacitor ADCs provide efficient high speed conversion of analog signals to digital signals. A representative switched capacitor ABC is shown at 10 in FIG. 1, in the form of a multi-stage pipelined ADC. As seen there, ADC 10 includes multiple stages, such as stages 11 and 12, each providing one or more bits of digital data to a digital correction circuit 15, which resolves the digital output from each stage into an overall digital output 16 that corresponds to an analog input 17. Each stage is a switched capacitor circuit operating in response to clock signals such as xcfx861 and xcfx862 and comparing an analog voltage input to thresholds based on reference signals Vrefp and Vrefn so as to produce the digital outputs as well as a residual analog signal. The residual analog signal is provided as input to the subsequent stage.
For proper operation of ADC 10, generating are needed for phase and timing signals as well as for reference voltages. There are shown respectively at 20 and 30 of FIG. 1. Thus, generator 20 for phase and timing signals generates clock signal xcfx861 for use during the sample phase of multiple stages 11 and 12, as well as clock signal xcfx862 for use during the amplification phase of multiple stages 11 and 12. Likewise, generator 30 generates reference voltages Vrefp and Vrefn for use by multiple stages 11 and 12. The forces of the present application is on the generator 30 for the reference voltages.
FIG. 2 shows a conventional generator 30 for generating references voltage Vrefp; a similar circular shown schematically at 31 is used to generate reference voltage Vrefn. As shown in FIG. 2, generator 30 includes a follower 32 connected between voltage source V+ and a current source 35 which, in turn, is connected to ground. Follower 32 is driven at its gate side by amplifier 34, which is connected in negative feedback relationship using a reference voltage Vref as a reference and the output Vrefp as negative feedback. With this arrangement, follower 32 is driven by amplifier 34 so as to provide as output Vrefp with good current capabilities stabilized through negative feedback at a voltage level corresponding to Vref.
Problems arise, however, in use of generators in the form shown at 30. For example, due to higher frequency switching of generator 30, and due to noise/glitches generated from MDACs and capacitors which are connected to the reference generator 30, the amplifier 34 (FIG. 2) needs to be very fast, such that it can react quickly to the noise and reset Vrefp to an ideal value (e.g., preferably within a fraction of a clock period). However, this would be difficult to achieve for high speed ADCs. Also, amplifier thermal noise would be high in such cases, which would make the Vrefp signal noisy.
An alternative is to design a low bandwidth amplifier to slowly servo Vrefp, and to use an external capacitor (e.g., with a sufficiently large capacitance) to lower the impedance seen by the reference at high frequencies. This alternative may minimize switching glitches and noise, but it also required extra circuitry, and for example, an extra, pin.
Another problem involves the value of Vrefp relative to the source voltage V+. Specifically, because a voltage drop Vgs exists between the gate and source of follower 32, and because it is not possible for amplifier 34 to output a voltage greater than the supply voltage V+, the value of Vrefp must be lower than V+ by at least an amount equal to Vgs. Typically, Vgs is around 1v, and for adequate design margins, Vrefp is typically set to a value 1.5 v less than source voltage V+. This amount of voltage drop, however, is wasteful and unnecessarily limits the dynamic conversion range of multiple stages 11 through 12.
It is an object of the present invention to address the foregoing through the provision of an improved generator for reference voltage signals used in a switched capacitor ADC.
In its most preferred form, a generator for reference voltages signals according to the invention is shown at 100 in FIG. 3. As seen there, the generator includes a follower 132 connected between a voltage source V+ and a current source 134 connected in turn to ground, as well as an amplifier 134 connected in negative feedback relationship with a reference voltage. Negative feedback to amplifier 134 is provided from the output of follower 132 (which forms the reference voltage signals Vrefp or Vrefn that are supplied to the ADC) through a switched capacitor filter 200.
The output of amplifier 134 is provided to a current sink 300 which drives the gate of follower 132. Current sink 300 has an effective resistance whose value is low relative to that of other components in generator 100, thereby providing a path to sink current through follower 132 and thereby providing increased rejection of noise.
Source voltage for current sink 300 is provided through charge pump 400. Charge pump 400 operates to increase the effective voltage level of supply voltage V+ for use by current sink 300, thereby allowing a design in which reference voltages for ADC 10 (such as Vrefp and Vrefn) are set very nearly equal to supply voltage V+ in spite of the voltage drop Vgs of follower 132.
Although in its preferred form all three components (i.e., charge pump 400, current sink 300 and switched capacitor filter 200) are used in the construction of a generator for reference voltages, it is possible to use fewer than all three components, such as only one or two components.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more compete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.